This invention pertains to a system for measuring the flow rate of a nuclear reactor primary coolant. More particularly, this invention pertains to a system which utilizes the decay of Nitrogen-16 activity in the coolant for flow measuring purposes.
There are existing flow measuring systems which utilize the decay of Nitrogen-16 activity present in the coolant loop to determine the flow rate. Such existing flow monitors have employed two Nitrogen-16 detectors, spaced along the coolant pipe. The detectors respond to the Nitrogen-16 activity as the coolant tranverses the pipe between the detector locations. The downstream detector reads less than the upstream detector due to the decay of the Nitrogen-16 activity as the water progresses between the detector locations. With decreasing flow, the relative difference between readings increases. The ratio of these readings is given generally by the following well-known equation: ##EQU1## where: V = the velocity, L = the separation distance between the two detectors 1 and 2, lambda is the decay constant of Nitrogen-16, and A.sub.1 and A.sub.2 are the respective activity readings at positions 1 and 2 by detectors 1 and 2 where position 1 is upstream of position 2. Such measuring systems have the disadvantage that the available distance between the Nitrogen-16 detectors is limited to a few feet on the hot leg of the coolant loop. Thus, the difference in magnitude between the two detector readings is in the order of 1 percent. Therefore, highly stable, sensitive, accurate detecting instruments have to be used and the sensivitity of the detectors 1 and 2 should be as close to being identical as possible. The greater the dissimilarity between the sensitivity of the two detectors, the greater is the error in the resultant velocity determination. Accordingly, in order to obtain a velocity determination which is accurate to within 1 or 2 percent, the counter identity must hold true to within approximately 10.sup..sup.-4 ; a task very difficult to achieve over large time intervals.
Thus, it is the object of this invention to provide apparatus that will more accurately determine the flow rate of the coolant in the nuclear reactor coolant loop by eliminating the counter identity problem.